[lldb] Remove ProcessRunLock::TrySetRunning

[JDevlieghere]

I traced the issue reported by Caroline and Pavel in #134757 back to the call to ProcessRunLock::TrySetRunning. When that fails, we get a somewhat misleading error message:

process resume at entry point failed: Resume request failed - process still running.

This is incorrect: the problem was not that the process was in a running state, but rather that the RunLock was being held by another thread (i.e. the Statusline). TrySetRunning would return false in both cases and the call site only accounted for the former.

Besides the odd semantics, the current implementation is inherently race-y and I believe incorrect. If someone is holding the RunLock, the resume call should block, rather than give up, and with the lock held, switch the running state and report the old running state.

This patch removes ProcessRunLock::TrySetRunning and updates all callers to use ProcessRunLock::SetRunning instead. To support that, ProcessRunLock::SetRunning (and ProcessRunLock::SetStopped, for consistency) now report whether the process was stopped or running respectively. Previously, both methods returned true unconditionally.

The old code has been around pretty much pretty much forever, there's nothing in the git history to indicate that this was done purposely to solve a particular issue. I've tested this on both Linux and macOS and confirmed that this solves the statusline issue.

A big thank you to Jim for reviewing my proposed solution offline and trying to poke holes in it.

[llvmbot]

@llvm/pr-subscribers-lldb

Author: Jonas Devlieghere (JDevlieghere)

Changes

I traced the issue reported by Caroline and Pavel in #134757 back to the call to ProcessRunLock::TrySetRunning. When that fails, we get a somewhat misleading error message:

> process resume at entry point failed: Resume request failed - process still running.

This is incorrect: the problem was not that the process was in a running state, but rather that the RunLock was being held by another thread (i.e. the Statusline). TrySetRunning would return false in both cases and the call site only accounted for the former.

Besides the odd semantics, the current implementation is inherently race-y and I believe incorrect. If someone is holding the RunLock, the resume call should block, rather than give up, and with the lock held, switch the running state and report the old running state.

This patch removes ProcessRunLock::TrySetRunning and updates all callers to use ProcessRunLock::SetRunning instead. To support that, ProcessRunLock::SetRunning (and ProcessRunLock::SetStopped, for consistency) now report whether the process was stopped or running respectively. Previously, both methods returned true unconditionally.

The old code has been around pretty much pretty much forever, there's nothing in the git history to indicate that this was done purposely to solve a particular issue. I've tested this on both Linux and macOS and confirmed that this solves the statusline issue.

A big thank you to Jim for reviewing my proposed solution offline and trying to poke holes in it.

---

Full diff: https://github.com/llvm/llvm-project/pull/135455.diff

4 Files Affected:

• (modified) lldb/include/lldb/Host/ProcessRunLock.h (+6-1)
• (modified) lldb/source/Host/common/ProcessRunLock.cpp (+4-14)
• (modified) lldb/source/Host/windows/ProcessRunLock.cpp (+4-12)
• (modified) lldb/source/Target/Process.cpp (+21-38)

diff --git a/lldb/include/lldb/Host/ProcessRunLock.h b/lldb/include/lldb/Host/ProcessRunLock.h
index b5b5328b4a33f..c83cab53a9a65 100644
--- a/lldb/include/lldb/Host/ProcessRunLock.h
+++ b/lldb/include/lldb/Host/ProcessRunLock.h
@@ -29,8 +29,13 @@ class ProcessRunLock {
 
   bool ReadTryLock();
   bool ReadUnlock();
+
+  /// Set the process to running. Returns true if the process was stopped.
+  /// Return true if the process was running.
   bool SetRunning();
-  bool TrySetRunning();
+
+  /// Set the process to stopped. Returns true if the process was stopped.
+  /// Returns false if the process was running.
   bool SetStopped();
 
   class ProcessRunLocker {
diff --git a/lldb/source/Host/common/ProcessRunLock.cpp b/lldb/source/Host/common/ProcessRunLock.cpp
index da59f40576978..f9bde96ae8ac9 100644
--- a/lldb/source/Host/common/ProcessRunLock.cpp
+++ b/lldb/source/Host/common/ProcessRunLock.cpp
@@ -37,21 +37,10 @@ bool ProcessRunLock::ReadUnlock() {
 
 bool ProcessRunLock::SetRunning() {
   ::pthread_rwlock_wrlock(&m_rwlock);
+  bool was_stopped = !m_running;
   m_running = true;
   ::pthread_rwlock_unlock(&m_rwlock);
-  return true;
-}
-
-bool ProcessRunLock::TrySetRunning() {
-  bool r;
-
-  if (::pthread_rwlock_trywrlock(&m_rwlock) == 0) {
-    r = !m_running;
-    m_running = true;
-    ::pthread_rwlock_unlock(&m_rwlock);
-    return r;
-  }
-  return false;
+  return was_stopped;
 }
 
 bool ProcessRunLock::SetStopped() {
@@ -60,6 +49,7 @@ bool ProcessRunLock::SetStopped() {
   ::pthread_rwlock_unlock(&m_rwlock);
   return true;
 }
-}
+
+} // namespace lldb_private
 
 #endif
diff --git a/lldb/source/Host/windows/ProcessRunLock.cpp b/lldb/source/Host/windows/ProcessRunLock.cpp
index 693641e42ed73..9f144b4c918f8 100644
--- a/lldb/source/Host/windows/ProcessRunLock.cpp
+++ b/lldb/source/Host/windows/ProcessRunLock.cpp
@@ -58,24 +58,16 @@ bool ProcessRunLock::ReadUnlock() { return ::ReadUnlock(m_rwlock); }
 
 bool ProcessRunLock::SetRunning() {
   WriteLock(m_rwlock);
+  bool was_stopped = !m_running;
   m_running = true;
   WriteUnlock(m_rwlock);
-  return true;
-}
-
-bool ProcessRunLock::TrySetRunning() {
-  if (WriteTryLock(m_rwlock)) {
-    bool was_running = m_running;
-    m_running = true;
-    WriteUnlock(m_rwlock);
-    return !was_running;
-  }
-  return false;
+  return was_stopped;
 }
 
 bool ProcessRunLock::SetStopped() {
   WriteLock(m_rwlock);
+  bool was_running = m_running;
   m_running = false;
   WriteUnlock(m_rwlock);
-  return true;
+  return was_running;
 }
diff --git a/lldb/source/Target/Process.cpp b/lldb/source/Target/Process.cpp
index a9787823b9108..cd5b8c1e8a6fa 100644
--- a/lldb/source/Target/Process.cpp
+++ b/lldb/source/Target/Process.cpp
@@ -577,9 +577,7 @@ void Process::Finalize(bool destructing) {
   // contain events that have ProcessSP values in them which can keep this
   // process around forever. These events need to be cleared out.
   m_private_state_listener_sp->Clear();
-  m_public_run_lock.TrySetRunning(); // This will do nothing if already locked
   m_public_run_lock.SetStopped();
-  m_private_run_lock.TrySetRunning(); // This will do nothing if already locked
   m_private_run_lock.SetStopped();
   m_structured_data_plugin_map.clear();
 }
@@ -1325,9 +1323,9 @@ void Process::SetPublicState(StateType new_state, bool restarted) {
 Status Process::Resume() {
   Log *log(GetLog(LLDBLog::State | LLDBLog::Process));
   LLDB_LOGF(log, "(plugin = %s) -- locking run lock", GetPluginName().data());
-  if (!m_public_run_lock.TrySetRunning()) {
-    LLDB_LOGF(log, "(plugin = %s) -- TrySetRunning failed, not resuming.",
-             GetPluginName().data());
+  if (!m_public_run_lock.SetRunning()) {
+    LLDB_LOGF(log, "(plugin = %s) -- SetRunning failed, not resuming.",
+              GetPluginName().data());
     return Status::FromErrorString(
         "Resume request failed - process still running.");
   }
@@ -1342,8 +1340,8 @@ Status Process::Resume() {
 Status Process::ResumeSynchronous(Stream *stream) {
   Log *log(GetLog(LLDBLog::State | LLDBLog::Process));
   LLDB_LOGF(log, "Process::ResumeSynchronous -- locking run lock");
-  if (!m_public_run_lock.TrySetRunning()) {
-    LLDB_LOGF(log, "Process::Resume: -- TrySetRunning failed, not resuming.");
+  if (!m_public_run_lock.SetRunning()) {
+    LLDB_LOGF(log, "Process::Resume: -- SetRunning failed, not resuming.");
     return Status::FromErrorString(
         "Resume request failed - process still running.");
   }
@@ -2718,13 +2716,8 @@ Status Process::LaunchPrivate(ProcessLaunchInfo &launch_info, StateType &state,
   SetPublicState(eStateLaunching, restarted);
   m_should_detach = false;
 
-  if (m_public_run_lock.TrySetRunning()) {
-    // Now launch using these arguments.
-    error = DoLaunch(exe_module, launch_info);
-  } else {
-    // This shouldn't happen
-    error = Status::FromErrorString("failed to acquire process run lock");
-  }
+  m_public_run_lock.SetRunning();
+  error = DoLaunch(exe_module, launch_info);
 
   if (error.Fail()) {
     if (GetID() != LLDB_INVALID_PROCESS_ID) {
@@ -2989,17 +2982,12 @@ Status Process::Attach(ProcessAttachInfo &attach_info) {
       if (wait_for_launch) {
         error = WillAttachToProcessWithName(process_name, wait_for_launch);
         if (error.Success()) {
-          if (m_public_run_lock.TrySetRunning()) {
-            m_should_detach = true;
-            const bool restarted = false;
-            SetPublicState(eStateAttaching, restarted);
-            // Now attach using these arguments.
-            error = DoAttachToProcessWithName(process_name, attach_info);
-          } else {
-            // This shouldn't happen
-            error =
-                Status::FromErrorString("failed to acquire process run lock");
-          }
+          m_public_run_lock.SetRunning();
+          m_should_detach = true;
+          const bool restarted = false;
+          SetPublicState(eStateAttaching, restarted);
+          // Now attach using these arguments.
+          error = DoAttachToProcessWithName(process_name, attach_info);
 
           if (error.Fail()) {
             if (GetID() != LLDB_INVALID_PROCESS_ID) {
@@ -3060,17 +3048,13 @@ Status Process::Attach(ProcessAttachInfo &attach_info) {
   if (attach_pid != LLDB_INVALID_PROCESS_ID) {
     error = WillAttachToProcessWithID(attach_pid);
     if (error.Success()) {
+      m_public_run_lock.SetRunning();
 
-      if (m_public_run_lock.TrySetRunning()) {
-        // Now attach using these arguments.
-        m_should_detach = true;
-        const bool restarted = false;
-        SetPublicState(eStateAttaching, restarted);
-        error = DoAttachToProcessWithID(attach_pid, attach_info);
-      } else {
-        // This shouldn't happen
-        error = Status::FromErrorString("failed to acquire process run lock");
-      }
+      // Now attach using these arguments.
+      m_should_detach = true;
+      const bool restarted = false;
+      SetPublicState(eStateAttaching, restarted);
+      error = DoAttachToProcessWithID(attach_pid, attach_info);
 
       if (error.Success()) {
         SetNextEventAction(new Process::AttachCompletionHandler(
@@ -5882,10 +5866,9 @@ void Process::ClearPreResumeAction(PreResumeActionCallback callback, void *baton
 }
 
 ProcessRunLock &Process::GetRunLock() {
-  if (m_private_state_thread.EqualsThread(Host::GetCurrentThread()))
+  if (Process::CurrentThreadIsPrivateStateThread())
     return m_private_run_lock;
-  else
-    return m_public_run_lock;
+  return m_public_run_lock;
 }
 
 bool Process::CurrentThreadIsPrivateStateThread()

[jimingham]

This looks good to me, but we've discussed this already so I'm a prejudiced judge. We should maybe wait on a second opinion...

[labath]

I was looking at this code as well, and I've (also) came to the conclusion that TrySetRunning is a problem.

[labath]

This should also do the was_running dance.

[labath]

Suggested change

	/// Set the process to stopped. Returns true if the process was stopped.
	/// Returns false if the process was running.
	/// Set the process to stopped. Returns true if the process was running.
	/// Returns false if the process was stopped.

(at least, that's what the windows implementation does -- and I think it makes sense)

[labath]

Thanks for looking into this.